Miniature pumps, hereafter referred to as micropumps, can be constructed using fabrication techniques adapted from those applied to integrated circuits. Such fabrication techniques are often referred to as micromachining. Micropumps are in great demand for environmental, biomedical, medical, biotechnical, printing, analytical instrumentation, and miniature cooling applications. Just as in larger applications, various pump designs are required for different micropump systems.
The valve components of micropumps may include passive polysilicon check valves, gas-controlled valves with silicon membranes, solenoid-actuated valves with nickel diaphragms, and magnetically or electro-statically driven control valves. Valves that include components that are actuated or otherwise driven can be characterized as active valves. Manufacture and operation of active valves can add substantial complexity and cost to the production of micropumps.
Passive-type valves, such as those having movable polysilicon check valves, can be manufactured with reduced complexity, although these valves can fail when the pumped fluid includes particulates. In this regard, the particulate sizes are of the same order of magnitude as the passages in the micropumps. The passive valves become obstructed by particulates and are, therefore, unable to provide a sufficient seal when required. As a result, such valves have limited effectiveness when employed for pumping fluids that include particulates.
Similarly, active valves that employ substantially rigid sealing membranes or diaphragms are susceptible to seal failure when used to pump fluids containing particulates. The particulates become embedded in the sealing surface as a result of the relatively high pressure applied to the rigid diaphragm as needed to ensure a seal with such a valve. Once the particulates become embedded in the sealing surface, the valve is thereafter prevented from fully closing.
Fixed valves are valves having no moving parts. Fixed valves represent the utmost simplicity and high reliability for pumping fluids. Such valves, which do not include parts that periodically seal and move apart, are especially advantageous for micropump systems used for pumping fluids that include particulates. Moreover, fixed-valve pumps are particularly useful for biological applications that require pumping fluids that contain cells. The cells are not damaged by the fixed valve pumps, as would otherwise occur in moving-parts valves.
The effectiveness of fixed valves can be characterized by the parameter "diodicity," which is the ratio of pressure drop in the reverse-direction fluid flow through the valve to the pressure drop in the forward-direction fluid flow through the valve, for a given flow rate. A basic design consideration for a fixed valve micropump is to develop valve configurations that result in a diodicity greater than 1.0. In this regard, the small size of such valves, and the very low flow range (100 nl/min to 50 ml/min, for example) will typically yield a relatively low Reynolds number, which number is a dimensionless parameter that is proportional to the product of the valve size and flow velocity. Accordingly, the valve configurations must effect the requisite diodicity in flows characterized by low Reynolds numbers, where flow separation and turbulence (with attendant significant pressure losses) are unlikely to occur.
U.S. Pat. No. 1,329,559 discloses a fixed valve that is designated as a"valvular conduit." The conduit is provided with enlargements, recesses, and projections that are said to offer virtually no resistance to the passage of fluids in one direction, yet provide a nearly impassible barrier to fluid flow in the opposite direction. When an oscillating flow of fluid is applied to one end of the conduit, the conduit acts as a one-way valve or fluidic diode, thereby permitting the oscillated or pulsed fluid to be pumped through the conduit. The conduit is mounted to a piston that is rapidly reciprocated to provide the pulsed flow of fluid through the conduit.
The valvular conduit described in U.S. Pat. No. 1,329,559 is full-sized, constructed of metal, and used for delivering fluids with flows that can most likely be characterized as having a relatively high Reynolds number. No insight is provided in that patent as to how such a conduit could be adapted to a micropump system and flows characterized by low Reynolds numbers.
The present invention is generally directed to the fabrication of a micropump that employs fixed inlet and outlet valves. In one preferred embodiment of the invention, the micropump is fabricated by micromachining techniques and operates as a diaphragm pump driven by a piezoelectric actuator and having fixed valves that effect the necessary diodicity of greater than 1.0.
As one aspect of the invention, a laser-assisted chemical etching fabrication technique is employed for providing smooth-walled, curved configurations of portions of the fixed valves. A fabrication technique known as LIGA may also be employed.
As another aspect of the invention, the fixed valves include discrete branches for substantially redirecting the reverse flow therethrough, thereby generating the pressure loss differences requisite for a valve diodicity of greater than 1.0.
As another aspect of the invention, the fixed valves include diffuser and nozzle configurations for generating the relatively high pressure losses in the reverse-flow direction.
The present invention can be implemented in a variety of ways. For example, a micropump fabricated by surface or bulk micromachining techniques can be connected to a fluidic circuit in which is incorporated a fixed valve of the present invention.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.